You can install llvm using brew since it now includes openmp.
brew install llvm
You can make a symlink if you want
ln -s /usr/local/opt/llvm/bin/clang /usr/local/bin/clang-omp
My makefile looks like this
CPP = /usr/local/opt/llvm/bin/clang
CPPFLAGS = -I/usr/local/opt/llvm/include -fopenmp
LDFLAGS = -L/usr/local/opt/llvm/lib
example: example.c
$(CPP) $(CPPFLAGS) $^ -o $@ $(LDFLAGS)